Bridged Ethernet networks consist of a number of Ethernet segments connected via Ethernet bridges. An Ethernet bridge has a number of Ethernet ports connected to Ethernet segments on which frames are received and transmitted, as well as a switch to couple frames received on an input port to one or more output ports. The function of an Ethernet bridge is to switch Ethernet frames between segments. Each Ethernet frame includes the source MAC address of the sending node and the destination MAC address of the receiving node. The behavior of an Ethernet bridge is described in more detail in the Institute of Electrical and Electronic Engineers (IEEE) Standard 802.1D, which is incorporated herein by reference in its entirety.
Depending upon the topology of a desired network, a network designer may be required to use two Ethernet bridges even though a single Ethernet bridge would suffice in terms of the number of Ethernet ports needed. For example, referring to FIG. 1, two disjoint networks are shown, each disjoint network consisting of three nodes. A first network comprises nodes A and C, which are connected via switch B. A second network comprises nodes D and F, which are connected via switch E. In this example, assuming that the switches B and E each have four or more Ethernet ports, it would seem that a single switch could be used to replace the two switches B and E.
Unfortunately, a conventional Ethernet bridge does not support this functionality in an appropriate manner. Specifically, assuming bridges B and E are replaced by a single bridge G, such as depicted in FIG. 2, traffic from node A will arrive at its destination of node C. However, such traffic may also arrive at nodes D or F. This routing of traffic to non-destination nodes may be unacceptable for reasons of network performance, network security and the like.
It is possible to separate a bridge into groups by using virtual local area networks (VLANs), where each VLAN is associated with a respective identifier (VLAN ID). Referring to FIG. 1, if one VLAN ID (denoted as X) is used for the traffic running between nodes A and C through bridges B and E, and another VLAN ID (denoted as Y) is used for the traffic running between nodes D and F through bridges B and E, then a substitution of bridges B and E by a single bridge G is possible. Bridge G blocks its ports to D and F for traffic associated with VLAN ID X, and block its ports to A and C for traffic associated with VLAN ID Y. It is also noted that VLANs may be employed to realize hierarchical spanning trees. This requires different VLANs to be used in the different groups (i.e., different domains), and to run a spanning tree per group of VLANs in each domain. To interconnect them one needs VLAN translation over a link, if the domains are connected by links, or VLAN translation within a node.
Unfortunately, these alternative solutions based on the use of VLANs require careful provisioning of the network, and put restrictions on the VLAN IDs used on different networks. This complexity limits the utility of the VLAN solution as the size and/or complexity of the network increases, such as with newer telecommunications networks.